Electric and hydraulic hybrid vehicle drive systems that comprise, next to a main drive, an electric or hydraulic auxiliary drive are generally known. However when applying such a hybrid drive system to drive all harvester functions it would result in a hydraulic or electric storage system that would have unmanageable dimensions and weights exceeding allowable limits. Therefor there have been proposed such electric or hydraulic hybrid vehicle drive systems of which the main drive, in the form of an internal combustion engine, is directly driving the main loads of the harvester and the electric or hydraulic auxiliary drive is used to drive one or more auxiliary loads.
EP2130735 describes one particular electric hybrid drive system for a harvester in which certain auxiliary loads, such as for example the unloading auger on a combine are driven by an auxiliary electric drive that is powered from a battery. This battery is charged by means of a generator mechanically coupled to the main drive in the form of an internal combustion engine. An auxiliary power control unit is provided that controls the power exchange between the internal combustion engine and the battery in function of the intermittent operation of for example the unloading auger. Such a system however presents several disadvantages, first of all each of the additional components for the electric drive system form a single point of failure. This means as soon as the motor or the generator or the battery fails the auxiliary load cannot be operated anymore. The battery and the generator further need to be dimensioned, often with a predetermined safety margin, to generate and store sufficient energy for the complete operating period of the auxiliary load, including any peak load, for example during startup of the unloading auger of a combine. Additionally, the number of intermittent operation periods the battery is able to perform, without unacceptable degradation is limited which affects the long term peak performance of the system, especially when the intermittent operating periods are repetitive and form an operating cycle with a frequency of for example a plurality of operating periods per hour. Finally, the addition of an electric motor for driving the auxiliary load represents a serious modification of the design of the harvester and makes it difficult to retrofit an existing combine harvester design with such a system as for example all auxiliary drives need to be provided with an electric motor.
Thus there still exists a need for a harvester comprising a hybrid vehicle drive system that overcomes the above mentioned disadvantages and allows for a system that is able to handle peak loads, such as for example during startup of an intermittent auxiliary load, efficiently and reliably.